Well this is my experience. before I had the new shoes for the LS, I was running the stock wheels, I was getting 17 to 18 MPG city drive, now I have the new shoes, 19's, it went down to 12 to 13 MPG city. I don't drive the car that much,, since I have a work car, I only drive the car twice a day to take and pick up my son from school, total of 3 miles a day. then running some errands in my area. I also reset the fuel MPG status and still same 12 to 13 MPG city.
Today I filled up the car and reset the fuel MPG status then I took a 4 hours drive, total of 230 miles and I got a better 23 to 24 MPG highway. Hopefully it stay's that way.
I always fill up a 93 octane premium since there is no 91 octane here in Houston. 87, 89, 93 octane.